Distribution of heavy metals in agricultural soils of the Hehuang Valley and assessment of human health risks: a case study of Xining City

This study addressed the ecological contamination and health risks associated with heavy metals in farmland soils under rapid urbanization. It evaluated the pollution characteristics and health risks of heavy metals (As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn) in surface soils from vegetable farmlands in the Xining region of the Hehuang Valley. The geo-accumulation index (I_geo) and potential ecological risk index (RI) were employed to assess the pollution level and ecological risk, respectively. The U.S. EPA health risk assessment model was applied to estimate non-carcinogenic risk (HI) and total carcinogenic risk (TCR) for adults and children via oral ingestion, dermal contact, and inhalation pathways. The positive matrix factorization (PMF) receptor model was applied for source apportionment. Thirty surface soil samples were analyzed. The result showed the following concentration order: Mn>Zn>Cr>As>Ni>Pb>Cu>Co>Cd. Among these, As (35.38 mg/kg), Cd (0.19 mg/kg), Ni (30.63 mg/kg), Pb (28.74 mg/kg) and Zn (72.15 mg/kg) exceeded the background values of Qinghai. As exhibited light to moderate pollution (I_geo=0.84). PMF analysis identified four sources: natural and agricultural mixed source (22.6%), industrial, residential and traffic mixed source (24.2%), agricultural source (23.9%), and traffic and natural mixed source (29.3%). The ecological risk index (RI = 82.52) indicated an overall low risk, though the potential risk of Cd required attention. Health risk assessment showed that the total non-carcinogenic risk for children (HI=1.21) was higher than that for adults (HI=0.47). The total carcinogenic risk (TCR=1.73×10⁻⁴ for children, TCR=1.25×10⁻⁴ for adults) did not exceed thresholds, but children exhibited significant exposure risks to Ni, and As. The study demonstrated that agricultural activities alongside industrial and traffic emissions were the main drivers of pollution in this farmland area. It was recommended to strengthen source control and farmland management measures for elements such as As, Cd, and Ni.